In performing for instance a flame-spraying process or a plasma-spraying process for providing a substrate with a highly corrosion and wear-resistant surface, the powder to be sprayed has to be continuously fed to the spray nozzle in a gas flow at a predetermined rate per time unit. Apparatus for performing a process of this type is already known from U.S. Pat. No. 3,517,861, this apparatus comprising a cylindrical powder reservoir, and a disc mounted above the bottom of the reservoir at an excentrically offset position relative thereto, the disc being formed with one or more circular arrays of perforations for receiving the powder from the reservoir therein. As a result of the excentrically offset mounting of the disc, a part of its circumferential portion projects outwards of the reservoir. At this location outward of the reservoir, the powder entrained in the perforations of the disc is introduced into the gas flow. To this purpose, an end of a gas supply passage opens above the disc at this location in alignment with a gas-and-powder discharge passage disposed below the disc. Although this arrangement permits the powder discharge rate to be reasonably accurately adjusted and to be varied within a relatively wide range, this apparatus suffers from the serious disadvantage that it is difficult to achieve a continuous and steady powder discharge flow when only small amounts of a given powder are to be sprayed.
If the powder to be sprayed may not come into contact with air, the powder which has not been consumed when changing from one powder to another one has usually to be discarded, because it is inevitable that the powder comes into contact with air when emptying the reservoir.
Also known already from U.S. Pat. No. 4,381,898 is a powder reservoir provided with an interior powder guide surface of a conical shape converging towards a pointed end adjacent a powder discharge passage at a bottom location. In this arrangement the discharge of the powder is brought about by maintaining the interior of the powder reservoir at an elevated pressure higher than that of the gas flow into which the powder is to be introduced. This pressure difference is also used for controlling the powder discharge rate. For enabling an apparatus of this type to also dispense powders having insufficient flow characteristics, the sidewalls of the interior powder guide surface are provided with gas entry ports for fluidizing the powder at a level above the powder outlet in the bottom area of the reservoir. With an apparatus of this type it is likewise not possible to controlledly dispense minimal amounts of a powder for the coating of smaller surfaces. This is because in an apparatus of this type the discharge of the powder is already started as soon as the gas for fluidizing the powder is supplied to the reservoir. Depending on the type of the powder, a start-up period of about 20 to 60 seconds is then required before the powder is steadily discharged at the desired rate.
It is therefore an object of the present invention to provide an apparatus of the type defined in the introduction, which permits small amounts of a powder to be sprayed at a desired steady rate, and which eliminates the necessity of discarding any considerable amounts of the powder when changing from one type of powder to another one.